1. Field of the Invention
The present invention relates generally to dendritic metal structures and devices including them. The present invention also relates particularly to methods for making dendritic metal structures without the use of solid electrolyte materials.
2. Technical Background
Electrically active structures find use in a variety of applications. One type of electrically active structure is a current generating structure, in which current is generated in response to an external stimulus. For example, solar cells are based on structures that generate current in response to absorption of solar radiation. This current can be collected to provide electrical energy. Similarly, optical sensors such as photodetectors can be based on current generating structures; absorption of an optical signal can generate current, which is electrically detected and can be correlated with an external stimulus. One type of common current generating structure is a p-n junction formed from a layer of p-type semiconductor in contact with a layer of n-type semiconductor. In such structures, absorbed light energy creates electron-hole pairs to generate current. Another example of an electrically active structure is a liquid crystal material, which changes its molecular alignment, and therefore its optical properties in response to an external field. Similarly, an electro-optic material can change its optical properties in response to an external field. Light emitting diodes (e.g., organic light emitting diodes) generate light in response to electrical energy. These electrically active structures are generally disposed between electrodes that collect the generated current or apply the external field.
However, electrode designs commonly used with electrically active structures suffer from a number of disadvantages. For example, conventional solar cells are formed from a current generating structure disposed on a bottom electrode and having a top electrode formed thereon. The top electrode is often formed as a series of wide bus bars with somewhat narrower branches extending between them. The dimensions of these electrodes (e.g., their line widths and the spaces between them) are generally large due to manufacturing cost limitations. The relatively large space between electrodes can create a high series resistance between the current generating sites and the top electrode, leading to inefficient energy collection from the areas of the current generating structure that are farthest from the electrodes. Packing the electrode structures more tightly is not a suitable solution, as a greater amount of electrode material will shield the current generating material from solar radiation, thereby rendering it useless for energy generation. Accordingly, using conventional techniques solar cell designers have to sacrifice efficient current collection in order to avoid blocking too much of the electrically active structure from light, leading to an inefficient use of the current collecting material, and therefore to lower energy generated per unit area. Similarly, liquid crystal and electro-optic devices require at least one electrode to allow a substantial amount of light to interact with the liquid crystal material and its tunable optical properties. There are transparent electrode materials, such as indium tin oxide, but these can be brittle, and prone to cracking when flexed.
Dendritic metal structures have been suggested for use as electrodes for use with electrically active structures. However, these designs can be improved. In some situations, for example, the solid electrolyte used to make the dendritic metal structures can be sufficiently light-absorbing to have an effect on final device performance.
Accordingly, there remains a need for electrode designs that can provide efficient electrical properties (e.g., current collection and field maintenance) while not blocking too much of the electrically active structure from light.